Towards climate-resilient restoration in mesic eucalypt woodlands: Characterizing topsoil biophysical condition in different degradation states

Investments in restoring native vegetation must increasingly allow for likely impacts of climate change, requiring re-evaluation of limits to ecological recovery and persistence. Nutrient enrichment and weed invasion are significant limits to restoration in mesic ecosystems, but in a drying climate, limits could shift towards more fundamental ecosystem functions. We used a state and transition framework to identify landuse-related changes in topsoil biophysical characteristics likely to influence climate resilience in mesic temperate eucalypt woodlands.

We compared topsoil condition in little-modified 'reference' states of the native ground-layer (dominated by tall tussock grasses) with four degraded ground-layer states identified in our state and transition framework. We hypothesized that 'nutrient-depleted' states (dominated by short tussock grasses) and 'nutrient-enriched' states (dominated by exotic annuals) would exhibit characteristics reflecting increased and decreased ecosystem vulnerability to a drying climate respectively.

Our hypothesis that nutrient-depleted states are more vulnerable to a drying climate was supported by their significantly slower soil-water infiltration rates and significantly lower levels of topsoil carbon, clay, micro-invertebrates, microbial activity and modeled water holding capacity than reference states. However, degradation was less pronounced beneath trees, and our prediction regarding enriched states was supported only for carbon.

Topsoil biophysical characteristics associated with different ground-layer states are predictable using a state and transition framework. Climate resilience of nutrient-depleted states appears compromised by topsoil biophysical degradation, indicating increasing need for attention in mesic ecosystems predicted to become drier under climate change.